Islamic State has executed 40 civilians in Mosul for "treason and collaboration", the United Nations has said.
The UN Human Rights office said the bodies of the victims were hung on electrical poles in the Iraqi city.
IS has also announced it has beheaded six of its own fighters for deserting the battlefield at Kokjali, near Mosul.
UN spokeswoman Ravina Shamdasani listed a series of alleged atrocities carried out by the group, including how one of their sources pretended to be dead during a mass execution of 50 former members of the Iraqi security forces.
She also said that a mass grave in Hamman al Alil containing more than 100 bodies was one of several IS "killing grounds", with bodies dumped in various locations.
Ms Shamdasani said IS is reportedly stockpiling large amounts of ammonia and sulphur and placing them in civilian locations in Mosul, possibly to be used as chemical weapons.
:: Iraqi forces make further advances into Mosul as liberation begins
As Iraqi forces work their way further into Mosul, public executions are being carried out, sometimes for using banned mobile phones as well as for desertion or collaborating.
She said people with explosive belts, thought to be teenagers or young boys, were being deployed in the alleyways of Old Mosul.
Women who have been abducted are being "distributed" to fighters or told they will be used to accompany IS convoys.
As many as 50,000 Iraqi soldiers, Kurdish Peshmerga fighters, Sunni Muslim Arabs and Shia Muslim militiamen launched an offensive on Mosul on 17 October.
It is being supported by US-led coalition warplanes and military advisers.
Troops have pushed into the east of the city, but it is expected to take weeks of fighting, if not months, to regain full control.
IS took over the city two years ago, driving Iraqi forces out as it seized a big part of western and northern Iraq.
The UN fears for the safety of up to 1.5 million civilians living in Mosul, after reports that thousands were being rounded up to be used as human shields.
Nearly 1,800 people were killed in and around the city in October, it said. More than 1,100 of them were civilians.